1. Field of the Invention:
This invention relates generally to correlating radiometer techniques, in combination with standard radar techniques for providing high resolution angular location and range gating measurements, and more particularly relates to polystatic cross correlating radar techniques useful for object angular location, ranging, and radial velocity and tangential velocity measurements for close targets, useful for automotive collision avoidance radar, cruise control radar, and self-mobile robotic systems.
2. Description of Related Art:
In general, conventional radar devices include a transmitting antenna emitting electromagnetic radiation generated by an oscillator, a receiving antenna, and an energy detecting receiver. The receiver provides a received radar signal to a signal processing unit where the radar signals are processed to detect and identify the presence of a target, and to determine its location and radial velocity with respect to the receiver. Distance can be determined by measuring the time taken for the radar signal to travel to the target and to return. The direction, or angular location of the target may be determined by the direction of arrival of the received radar signal. Directional information is usually obtained with narrow antenna beams, and the radial velocity of the target with reference to the receiver can be measured by detecting shifts in the carrier frequency of the radar signal reflected from the target, commonly known as the Doppler effect. Continuous waveforms can be used to take advantage of the Doppler frequency shift, and frequency or phase modulation of the continuous waveform permits range measurements from the received radar signals.
Modern radar typically uses a common antenna for both transmitting and receiving, known as monostatic radar. A bistatic radar is one in which the transmitting and receiving antennae are separated by a given distance. In early experimental radar systems this was known as CW wave-interference radar. Such early experimental radar systems utilized continuous waveform (CW) radar signals, and depended for detection upon interference produced between the signal received directly from the transmitter and the Doppler frequency shifted signal reflected by a target.
When several separate receivers are employed with a signal transmitter, the radar system is known as multistatic, or polystatic radar. An essential feature of the bistatic or polystatic radar is that the radiated signal from the transmitter arrives at the receivers from the scattered path which includes the target, and is also directly correlated with the receiver in a direct path from the transmitter. Information from the transmitted signal allows extraction of information from the scattered signal. Thus, from the transmitted frequency, the Doppler frequency shift, and the phase or time shift may also be determined. Although a bistatic radar can be operated with either pulse modulation or continuous waveform energy, continuous wave radar requires considerable isolation between the transmitter and receiver, which is obtainable in a bistatic or polystatic radar because of inherent separation between the transmitter and receivers.
Continuous wave radar also may be used for determining range if a timing mark is applied to the CW carrier, permitting the time of transmission and time of return to be recognized. Such a timing mark is applied to the CW carrier, permitting the time of transmission and time of return to be recognized. Such a timing mark can be used for identifying the transmitted carrier as well. A widely used technique to allow a broad spectrum of radar and timing information is frequency modulation of the carrier (FM-CW).
Another conventional radar technique for obtaining information from a received radar signal is the process of range gating. Each range gate opens sequentially just long enough to sample the received signal corresponding to a different range of time corresponding to a distance of travel of the signal in space.
If the bandwidth of the receiver pass bank is wide compared with that of the received signal energy, extraneous noise is introduced, reducing the signal to noise ratio of the received signal. If the receiver bandwidth is narrower than the bandwidth of the received signal, noise energy is reduced, along with a considerable portion of the received signal energy. This also reduces the signal to noise ratio. A matched filter functions to maximize the output peak signal to mean noise ratio. A matched filter receiver can be replaced by a cross correlation receiver that performs the same operation. In a cross correlation receiver, an input signal is multiplied by a delayed replica of the transmitted signal, and the product is passed through a low pass filter to perform integration of the signal. It would be desirable to combine such radar techniques to permit high resolution location and range and radial velocity measurements, as well as tangential velocity measurements of a close target. The present invention addresses this need.